polarization

Polarization is a condition in which electromagnetic waves are constrained to vibrate in a certain plane or planes (see polarized
light). Electromagnetic waves consist of vibrating electric and magnetic
fields, with the direction of vibration perpendicular to the direction of
motion of the wave. If the direction of vibration remains steady with time,
the wave is said to be 100% linearly polarized or plane
polarized in that direction. If the direction of vibration rotates
at the same frequency as the wave, the wave is said to be 100% circularly
polarized. Most naturally occurring electromagnetic waves have
a direction of vibration that jiggles around at random: these are said to
be unpolarized. Intermediate states, where there is some
jiggling around an average direction, are said to be partially polarized;
the amount of order is specified by the degree of polarization,
which ranges from 0 to 100%. Plane polarization is usually
caused by scattering, and circular polarization by strong magnetic fields.

Polarization explained

The vibrations in the rope will only pass through
the narrow gap in the railings if the vibrations are parallel to the
individual rails. Thus an up and down vibration is transmitted through
the vertical railings, but is stopped by the horizontal ones.

Light is a form of wave motion, but as it
travels at such a high speed (300,000 km/s) it isn't possible to observe
directly the way in which the moving waves vibrate. However, experiments
show that the waves vibrate at right angles to the direction of the light
beam, and because of this light is said to consist of transverse
waves.

Since the vibration of light waves can't be seen, it's useful to see how
other visible transverse waves vibrate. Such waves may be set up
in a length of rope by repeatedly moving one end up and down (i.e. at right
angles to the rope). The vibrations in the rope will pass through a narrow
gap provided the longest axis of the gap is parallel to the direction of
the vibrations. Thus if the rope is vibrating up and down, the motion will
be transmitted through a vertical gap. However, if the rope also passes
through a horizontal gap the vibrations can't pass through this.

The rope in this example can only vibrate in one plane (at right
angles to the direction of travel). But an ordinary beam of light consists
of a great number of individual waves which are vibrating in different directions
(all at right angles to the light path). But under certain conditions it
is possible for all the vibrations in a light beam to be confined to
one plane are right angles to the beam. Such a beam is said to be polarized.

Crystals that polarize

Certain crystals, in particular transparent crystals of calcite (a form of calcium carbonate) known as Iceland spar, have
the property of polarizing light. The ability of such crystals to polarize
light can be tested by passing a ray of light through two crystals of the
substance. Each crystal can be thought of as containing a number of very
narrow slits parallel to each other. These "slits" are so narrow that only
the waves whose vibrations are parallel to the slits can pass through.

Thus the light emerging from the first crystal is made up of waves whose
vibrations are almost parallel with one another. The passage of light whose
plane of polarization does not correspond with the favored plane is stopped.
If the second crystal is arranged so that the "slits" are parallel with
those in the first crystal the polarized light leaving the first crystal
will pass through the second crystal also. But if the second crystal is
then rotated, the amount of light transmitted will be reduced until a point
will be reached (at right angles to the first position) when no light is
transmitted through the second crystal. In this position the waves of polarized
light are vibrating at right angles to the slits, so the vibrations are
unable to pass through it.

Polarization by reflection

Light can be polarized by reflection as well as by refraction. In fact the
French physicist E. L. Malus (1775–1812) discovered this property
of light after observing sunlight reflected from windows. Malus' discovery
can best be illustrated by means of a simple experiment. Two mirrors are
arranged parallel to each other, forming a kind of periscope. When one of
the mirrors is rotated it is found that the amount of light reflected
from the second mirror gradually decreases to a minimum (it disappears completely
in one position – but this is very difficult to attain) and then increases
again. What is happening is that on reflection at the first mirror the vibrations
in all but one plane are absorbed. In other words, the reflected
beam is polarized. How completely the light is polarized depends on the angle of incidence. As the proportion
of light reflected from a surface is quite small, this is not a very effective
method of obtaining polarized light, so light is usually polarized by refraction.

Polaroid

One quite satisfactory polarizing medium has been manufactured and is marketed
under the Polaroid trade name. It comprises a cellulose nitrate film containing
very small crystals of iodoquinine sulfate. In manufacture, these tiny crystals
are made to line themselves up in one direction so that they behave like
crystals of Iceland spar.

Polaroid disks can be used to produce polarized light in much the same way
as with other polarizing media. If ordinary light enters the film, the light
emerging is polarized – almost all vibrations are right angles to the
favored direction have been absorbed. That the light has been polarized
can be shown by rotating a second Polaroid disk in the path of the light
emerging from the first. As the second disk is rotated a point is reached
at which where virtually no light is transmitted – the greatest amount
of light is transmitted when the disk is at right angles to this position.
Polaroid has many uses, not the least of which is sun glasses. With these
the glare caused by the sun's light being reflected from the road or water
surfaces, etc., is considerably reduced. The reflected light is already
polarized so that only that portion of the light which is polarized in the
favored direction is transmitted through the Polaroid disk.

Optical activity and the polarimeter

Solutions of various sugars, including cane sugar (sucrose) and of certain
other organic substances have the property of rotating the plane
of polarization of light passed through them. The extent of the
rotation is measured with an apparatus called a polarimeter or saccharimeter. This comprises a source of monochromatic light –
usually a sodium lamp, a means of polarizing the light (a polarizer),
a tube to hold the sample, and a means of checking the plane of polarization
(an analyzer).

To use the apparatus the tube is first filled with water, the lamp switched
on and the analyzer rotated until no light is seen to emerge from it. The
polarizer and analyzer are then said to be crossed. The water in
the tube is then replaced by the solution under test. The analyzer is again
rotated until it is again crossed, and the angle through which the plane
of polarization has been rotated is then noted. Some substances (e.g. d-glucose)
cause the plane of polarization to rotate in a clockwise or right-handed
sense. These are said to be dextrorotatory.
By contrast, levorotatory substances
(e.g. l-fructose) cause the plane of polarization to be rotated
in an anticlockwise or left-handed sense. Such information provides clues
to the structure of these compounds. The amount of rotation indicates the
strength of the solution.